1. Field of the Invention
The present invention relates to a water electrolysis cell for producing hydrogen and oxygen by electrolyzing water.
2. Description of the Related Art
Heretofore, there has been known, as shown in FIG. 1 of the accompanying drawings, a water electrolysis cell 1 for producing hydrogen and oxygen by electrolyzing water, which comprises a solid polymer electrolyte membrane 2 sandwiched between a pair of catalytic layers 3, 4 and a pair of porous current distributors 5, 6 mounted respectively on the catalytic layers 3, 4.
Each of the catalytic layers 3, 4 is produced by preparing an electrolytic solution of an electrolyte made of the same components as the solid polymer electrolyte membrane 2, dispersing a given amount of catalyst powder in the electrolytic solution to produce a paste, and applying the paste to a sheet of polytetrafluoroethylene according to a screen printing process. The solid polymer electrolyte membrane 2 is sandwiched between the catalytic layers 3, 4 on those sheets of polytetrafluoroethylene. The assembly is hot-pressed, transferring the catalytic layers 3, 4 to the solid polymer electrolyte membrane 2. In this manner, the catalytic layers 3, 4 are joined to the solid polymer electrolyte membrane 2. The catalytic layers 3, 4 function as anode and cathode catalysts, respectively.
The water electrolysis cell 1 operates as follows: The catalytic layers 3, 4 and the porous current distributors 5, 6 are supplied with water. When a voltage is then applied to the water electrolysis cell 1, water is electrolyzed at the anode according to the formula (1) shown below, generating oxygen and hydrogen ions, with electrons given to the anode. The hydrogen ions pass through the solid polymer electrolyte membrane 2 to the cathode, and are given electrodes from the cathode. As a result, hydrogen is generated at the cathode according to the formula (2) shown below.2H2O→O2↑+4H++4e−  (1) 2H++2e−→H2↑  (2) 
In the water electrolysis cell 1, therefore, oxygen can be generated from the anode, whereas hydrogen can be generated from the cathode.
One known catalyst for use in the anode of the water electrolysis cell 1 is an iridium-based catalyst made of iridium, a binary alloy thereof, or an oxide thereof. The iridium-based catalyst is generally stable under conditions for water electrolysis and hence is suitable for use as a catalyst for water electrolysis. However, the iridium-based catalyst is problematic in that it fails to provide a sufficiently low oxygen overvoltage, reducing the energy efficiency.
To solve the above problem, it may be proposed to use a ruthenium-based catalyst made of ruthenium, a binary alloy thereof, or an oxide thereof which can provide an oxygen overvoltage lower than the iridium-based catalyst. Since, however, the ruthenium-based catalyst generally tends to be eluted under the water electrolysis conditions, water electrolysis cells which use the ruthenium-based catalyst as the catalyst at the anode have a relatively short service life, and cannot remain operational for a long period of time.
There have been proposed mixed catalysts comprising a mixture of an iridium-based catalyst and a ruthenium-based catalyst. For example, Japanese laid-open patent publication No. 10-273791 discloses a catalyst which comprises a mixture of ruthenium oxide and iridium oxide. It is stated in the publication that a water electrolyte cell which uses the disclosed catalyst has a cell voltage in electrolysis lower than a cell using the iridium-based catalyst.
The disclosed mixed catalyst comprises a simple mixture of an iridium-based catalyst and a ruthenium-based catalyst. Therefore, when the ruthenium-based catalyst has been eluted with time, only the iridium-based catalyst which provides a higher oxygen overvoltage than the ruthenium-based catalyst remains as the catalyst, resulting in a reduction in the energy efficiency in electrolysis.